I. Field
The present invention relates generally to circuits, and more specifically to a voltage controlled oscillator (VCO).
II. Background
Wireless communication systems are widely deployed to provide various communication services such as voice, packet data, and so on. These systems may implement various wireless standards and may operate in various frequency bands that may be spaced far apart. A given system may also operate at different frequencies in different geographic areas (e.g., different countries).
A wireless device (e.g., a cellular phone or a handset) may need to operate at multiple frequency bands and support multiple standards in order to communicate with second and third generation wireless communication systems that are widely deployed throughout the world. The wireless device has at least one radio frequency (RF) front-end to generate an RF output signal for transmission via a wireless link and to process an RF input signal received via the wireless link. Each system/standard typically imposes stringent requirements on the RF output signal to ensure good performance. The tight specifications imposed by each system/standard normally prevent the use of a single RF front-end for all frequency bands and standards supported by the wireless device, which would be an optimum solution in terms of circuit area, complexity, and cost. Instead, a multi-band, multi-standard wireless device normally duplicates some circuit blocks or even an entire RF front-end for each supported standard and/or frequency band.
An RF front-end typically has at least one VCO to generate local oscillator (LO) signals used for frequency upconversion and downconversion. The VCO typically has an LC resonator composed of an inductor (L) and one or more capacitors (C). The oscillation frequency of the VCO, and hence the frequency of the LO signal, may be varied by adjusting a variable capacitor (varactor) within the LC resonator. The range of frequencies (or the tuning range) achieved with the varactor is typically small and is often used to account for variations in integrated circuit (IC) process, temperature, power supply, and so on.
To support multi-band operation, the LC resonator typically includes a bank of capacitors that may be selectively switched on or off to shift the nominal frequency of the VCO to different frequency bands. A VCO with a switch capacitor bank can achieve good performance when the frequency bands of interest are relatively close to each other, e.g., within 10 to 20% of each other. However, when the frequency bands are sufficiently far apart, a large range of capacitances is needed to tune the VCO to the different frequency bands. The large capacitance range translates to a large variation in the peak impedance of the LC resonator, which in turn causes a relatively large and undesirable variation in the VCO amplitude and phase noise over the different frequency bands.
A multi-band wireless device may employ multiple VCOs to support operation on multiple frequency bands. Each VCO may then be designed to achieve good performance for a specific frequency band. However, the use of multiple VCOs for multiple frequency bands increases cost, system complexity, and circuit area, all of which are undesirable.
There is therefore a need in the art for a multi-band VCO having good performance.